Malignant gliomablastomas (GBMs) are the most common and fatal brain tumors with no definitive treatment available to date. Recent studies identified a small population of cells (1-10%) with stem- like features in glioblastomas. We identified the Maternal Embryonic Leucine zipper Kinase (MELK) as a marker for normal hematopoietic and neural progenitors. In addition, we and others have implicated MELK in tumor growth and proposed it as a therapeutic target for GBMs. Despite the solid body of in vitro data suggesting that MELK is selectively upregulated in GBM compared to normal brain, no in vivo evidence is available to date establishing MELK expression as a marker for tumor-initiating cells or documenting its ubiquitous expression throughout the tumor. In this proposal we will investigate whether MELK plays role in GBM and whether MELK expression enriches for tumor-initiating cells in vivo. We will take advantage of MELK-GFP promoter-reporter to prospectively isolate MELK-high/-low cells by FACS and transplant them at limiting dilutions in recipient brain to determine their tumor-initiating capacity (Aim1). We will determine whether MELK catalytic function is required for tumorigenesis in vivo. (Aim 2). We will investigate whether the germ line ablation of MELK catalytic function (3MELK mice) eliminates/reduces experimental glioblastomas in the Ink4/Arf-null,/EGFRvIII mouse model. It is possible that various mechanisms compensate for the lack of MELK during development. To address this issue we will use a transplantation approach to determine whether knocking down MELK expression using shRNA will inhibit GBM formation in vivo. Finally, we will test small molecule inhibitors of MELK in vitro using primary human GBMs. We will validate 6 existing lead MELK inhibitors in primary human GBM lines and optimize the most potent MELK inhibitors using combinatorial/medicinal chemistry. The expression analysis of primary tumors identified hundreds of gene candidates. The critical remaining question is: which genes represent promising therapeutic targets? Validation of a particular protein to be a relevant and realistic target for treatment is literally a life-and-death decision. Given the success of Gleevec, Iressa, Tarceva, and other small molecule inhibitors of kinase catalytic activity, MELK as kinase presents a formidable opportunity for drug development. Although we and others previously proposed MELK as a unique drug target for GBM treatment, no in vivo validation has been demonstrated to date. The significance of this proposal is in validating MELK, in particular its catalytic activity, as a drug target for GBMs both in animal models using a genetic approach and in primary human GBM lines in culture using candidate small molecule inhibitors of MELK.